1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to a column driver of an LCD.
2. Discussion of the Related Art
One of the most important portions of a thin film transistor-liquid crystal display (TFT-LCD) is a digital-to-analog converter (DAC) which generates an output voltage corresponding to digital input data. The DAC includes a decoder switch that selectively outputs a voltage corresponding to one of a plurality of input voltages. The DAC applies sixty-four input voltages in order to display sixty four gray levels, and has sixty-four analog switches and decoders for the purpose of selecting one voltage corresponding to one of sixty four input data. Accordingly, if there are 240 output channels, then 64.times.240 analog switches are required, and an interconnection connecting sixty four input voltages to each analog switch becomes complicated.
FIG. 1 is a block diagram of a column driver of a conventional TFT-LCD. Referring to FIG. 1, the conventional column driver includes a control logic 11 having an address shift register, and a resistor string 12 for converting input voltage levels V0 to V8 into voltage levels V0 to V63. An input register 13 sequentially stores R, G, B data which are applied thereto when the shift register is shifted, and a storage register 14 stores the R, G, B data sequentially stored in input register 13 such that each of R, G, B data is stored at the same time. A DAC 15 compares sixty four voltages output from resistor string 12 according to the data output from storage register 14, and generates a voltage corresponding to the input data.
FIG. 2 is a block diagram of a conventional DAC corresponding to one channel. Referring to FIG. 2, the conventional DAC includes a level shifter 21 and a plurality of multiplexers 22. Level shifter 21 makes the level of data output from storage register 14 coincide with sixty four voltage levels V0 to V63 output from resistor string 12 of FIG. 1. A multiplexer 22 compares sixty four input voltages, and outputs a voltage corresponding to the input data. Multiplexer 22 uses a data signal output from level shifter 21 and data inverted from the data signal output of level shifter 21 as selection signals S0, S1, S2, S3, S4, S5, and S0, S1, S2, S3, S4, S5.
Now, the operation of the conventional LCD driver will be explained below. As shown in FIG. 2, multiplexer 22 sequentially compares two voltages from the sixty four input voltages with each other, and selectively outputs one voltage level corresponding to input data. Here, for the selection signals of multiplexer 22, the data signal and the inverted data signal that pass through the level shifter 21 are used. FIG. 3 is a block diagram of a conventional voltage interconnection. The voltage interconnection includes as many DACs as there are channels.
However, the conventional LCD driver has the following problems. When a plurality of channels are required, a plurality of DACs, each of which has a plurality of multiplexers, are also needed. Accordingly, the area occupied by the DACs becomes large, and thus the driver area also becomes large. Furthermore, the interconnection for connecting sixty four voltage levels to each DAC becomes complicated.